When hard coatings are formed on WC-based cemented carbide substrates by a physical vapor deposition method, the hard coatings are required to have high adhesion strength to the substrates. To improve adhesion strength between substrates and hard coatings, various methods have been proposed. JP 6-74497 B discloses a method for forming a hard ceramic coating by conducting the ion bombardment of metals on a substrate surface in vacuum, and then a melt deposition method. Also, Japanese Patents 4535249 and 4590940 disclose methods of ion-bombarding cemented carbide substrates of cutting tools with Cr in vacuum, and then forming predetermined hard coatings. Coatings having somewhat high adhesion strength are obtained by these methods, in which metal ions are bombarded in vacuum, but they fail to meet increasingly higher demand to the performance of cutting tools, etc., and further improvement is required.
JP 2009-220260 A discloses a method for producing a hard-coated tool by ion-bombarding metal Ti to a surface of a WC-based cemented carbide substrate in an Ar- or N2-based atmosphere to form a W-modified phase, forming a carbide phase immediately thereon, and forming a hard coating immediately on this carbide phase. However, the ion bombardment of Ti results in the formation of a decarburized phase under a modified phase, resulting in insufficient adhesion strength to a hard coating. Also, TiC formed on the modified phase does not have sufficient impact resistance, resulting in low adhesion strength.